A reciprocating well pump normally has a barrel secured to a string of production tubing. A piston located in the barrel strokes between a down stroke and an up stroke. A travelling valve mounted with the piston opens a bore of the piston during the down stroke to flow well fluid in the bore of the piston into the barrel above the piston. During the up stroke, the travelling valve closes to lift the well fluid from the barrel upward. A standing valve at an upper end of the barrel will be closed during the down stroke and opens on the op stroke to allow the well fluid being lifted to low up into the production tubing.
A string of rods may extend down the tubing for stroking the piston. Other types of stroking devices are known. For example, an electric motor may be installed below the barrel. A linear actuator coupled between the motor and the piston converts rotary motion of an output shaft of the motor into linear motion of the piston. The linear actuator may include a rotatable shaft with helical grooves engaged by a ball nut that is restrained from rotating. The rotation of the linear actuator shaft causes the ball nut to move up one set of helical grooves until reaching a crossover at the top of the linear actuator shaft, then move down a second set of helical grooves. A linking member connects the ball nut with the piston to move the piston in unison. The linear actuator shaft rotates in only one direction.
A dwell time occurs at or near the top of the stroke while the ball nut moves from one set of helical grooves to the other. During this dwell time, the standing valve will be open, communicating the weight of the column of well fluid in the production tubing with the well fluid in the barrel. Normally, the standing valve element is a ball that drops onto a seat due to gravity when the well fluid being pushed up by the piston stops flowing. In some instances, the well fluid pressure in the barrel is high enough to keep the standing valve element from closing, creating a stalled condition.